In the recent div3 contest, I submitted a code which passed the pretest during the contest but gave runtime error at the later checking. The strange thing is that when I submitted the same code again, the error was just gone

Contest Submission: 324871292
Later Submission: 325107819

There are no buffer overflows, all string accesses are bounds checked so this seems like a toolchain issue to me, even though how improbable that sounds

It seems that Valarrays should be quite useful in some problems due to the fact you can do entire operations on it in a single line. From what I've researched, while it is not optimized specially be any compiler, there is still no reason for it to be slower than a vector.

What are its downsides that seem to outweigh the convenience it provides?